(a) Field of the Invention
The present invention relates to a graphene composition having liquid crystalline properties and a preparation method thereof, and more particularly to a graphene composition wherein graphene having useful electrical properties is uniformly dispersed in a medium, whereby it is chemically and physically stable, exhibits a liquid crystal phase in a wide temperature range and has good compatibility with other compounds, and to a preparation method thereof.
(b) Background of the Related Art
Graphene is a single layer of sp2 hybrid carbon atoms covalently bonded with hexagonal arrangement, and is the basic building block for carbon nanotubes (CNTs), buckyballs, and graphite. Before it was found that graphene exists as a single layer, graphene was demonstrated to have half-integer quantum hall effects at room temperature, and high mechanical properties, and thus it has attention as a novel element.
The most noticeable property is that electrons flow in graphene as if they are weightless, which means that electrons flow at the velocity of light in vacuum. Another property of graphene is that it has an unusual half-integer quantum hall effect for electrons and holes.
The electron mobility of graphene known so far is from about 20,000 to 50,000 cm/Vs. Also, it is advantageous to use graphene since products made from graphene are inexpensive while products made from carbon nanotubes which are similar affiliations to graphene are expensive due to low yields obtained during synthesis and purification processes even though the carbon nanotubes are inexpensive themselves. In case of single wall carbon nanotubes, they exhibit different metallic and semiconducting characteristics according to their chirality and diameter. Furthermore, single wall carbon nanotubes having identical semiconducting characteristics have different energy band gaps depending on their chirality and diameter. Thus, single wall carbon nanotubes must be separated from each other in order to obtain desired semiconducting or metallic characteristics. However, it is known to be very difficult to separate single wall carbon nanotubes from each other.
On the other hand, it is advantageous to use graphene that a device can be easily designed to exhibit desired electrical characteristics by arranging the crystalline orientation in a desired direction since electrical characteristics of graphene are changed according to the crystalline orientation. The characteristics of the graphene can be efficiently applied to carbonaceous electrical devices or carbonaceous electromagnetic devices.
Meanwhile, liquid crystals possess both the fluidity of liquid and the anisotropic properties of solids and liquid crystal phase is a mesophase between solid crystals having positional order and orientational order and liquid having moleculars arranged in a disordered state. Liquid crystals have orientational order while exhibiting fluidity like liquid.
A liquid crystal molecule is referred to as “mesogen” and the mesogen exhibits properties including optical anisotropy and dielectric anisotropy. Due to the dielectric anisotropy or shape of the mesogen, liquid crystals are readily arranged by an external field. When the mesogens are aligned in one direction by applying an external field, the anisotropy of each of the mesogens macroscopically appears, and the direction of polarized light is changed or the mechanical properties of the mesogens are changed.
Due to such properties, in many cases, liquid crystals are used to impart anisotropic properties to materials or to impart liquid crystalline properties to materials in order to control the orientation using an external field.
Typical examples thereof are liquid crystal displays (LCDs) and fibers. In the case of LCDs, the passage of light from a backlight unit can be controlled by changing the orientation of mesogens using an electric field. In the case of fibers, because the mechanical properties in the axial direction should be much higher than the mechanical properties in other directions, materials having liquid crystalline properties themselves can be made into fibers or made into composites with filler, thus improving the physical properties of the materials.
Accordingly, imparting liquid crystal properties to a material means that the orientation of the material can be easily controlled and materials having different mechanical, optical and electrical properties according to the orientation can also be made. Thus, imparting liquid crystal properties is important in terms of processes.
In order to impart liquid crystal properties to materials or improve the physical properties of liquid crystalline properties as described above, studies on the liquid crystalline properties of carbon nanotubes have been conducted (Shanju Zhang, Satish Kumar, Small Journal, 4:9, 1270˜4283, 2008), and studies on the nematic liquid crystalline properties of multi-wall carbon nanotubes have been conducted (Song, W., Kinloch I. A., Windle, A. H., Science, 302, 1363, 2003).
In addition, Korean Patent Registration No. 0663716 discloses a method of uniformly dispersing carbon nanotubes in a liquid crystalline material, and Korean Patent Registration No. 0829513 discloses a carbon nanotube-enhanced thermotropic liquid crystal polymer nanocomposite obtained by adding carbon nanotubes to a thermotropic liquid crystal polyester copolymer resin, and a preparation method thereof.
However, the above-described prior documents employ carbon nanotubes which show very low yields when purified after synthesis, exhibit different metallic and semiconducting characteristics according to their chirality and diameter, different energy band gaps, and must be separated from each other in order to obtain desired semiconducting or metallic characteristics. Particularly, Korean Patent Registration Nos. 0663716 and 0829513 merely improve the physical properties of liquid crystalline materials by dispersing carbon nanotubes in the liquid crystalline materials, and thus the use of the liquid crystal materials as carbon materials in various fields is limited.
Meanwhile, Japanese Patent Registration No. 3265719 discloses a liquid crystalline resin composition containing graphite in a liquid crystalline polyester and/or polyester amide forming an anisotropic melt phase.
However, Japanese Patent Registration No. 3265719 merely improves the physical properties of the liquid crystalline material by dispersing graphite in the liquid crystalline material, and thus the use of the liquid crystalline material to provide liquid crystalline properties in various fields is limited.
Accordingly, the present inventors have made many efforts to solve the above-described problems occurring in the art and, as a result, have found that, when graphene having useful electrical properties are uniformly dispersed in a medium, a liquid crystalline graphene composition is prepared which is chemically and physically stable, exhibits a liquid crystal phase in a wide temperature range and has good compatibility with electromagnetic materials, polymers and the like, thereby completing the present invention.